Proliferative disorders such as cancer are characterised by the uncontrolled growth of cells within the body. As such proliferative disorders generally involve an abnormality in the control of cell growth and/or division leading to the formation of tumour and ultimately death. Without wishing to be bound by theory it is thought that this is caused by the pathways that regulate cell growth and division being altered in cancer cells. The alteration is such that the effects of these normal regulatory mechanisms in controlling cell growth and division either fails or is bypassed.
The uncontrolled cell growth and/or division ultimately proves fatal for the patient as successive rounds of mutations on the part of the cell then typically lead to the cancer cells having a selective advantage over normal healthy cells in the body of the patient leading to the cancer cells predominating in the cell mass of the patient. The cancer cells then typically metastasize to colonize other tissues or parts of the body other than the part of origin of the cancer cell leading to secondary tumours which eventually lead to organ failure and the death of the patient. It is the difficulty in controlling the rapid cell growth and division that is characteristic of cancer cells that make it hard to come up with effective chemotherapeutic strategies.
A number of traditional treatments for proliferative disorders such as cancer seek to take advantage of their higher proliferative capacity and thus their higher sensitivity to DNA damage. Treatments that have been utilised include ionizing radiation (γ-rays, X-rays and the like) as well as cytotoxic agents such as bleomycin, cis-platin, vinblastine, cyclophosphamide, 5′-fluorouracil and methotrexate. These treatments all rely on causing damage to DNA and destabilisation of the chromosomal structure eventually leading to death of the cancer cells.
The problem with many of these approaches is that they are non-selective for cancer cells and healthy cells can and often will be adversely affected by the treatment. This is hardly surprising given that the cellular mechanisms targeted by these strategies occur in healthy cells as well as in cancer cells (although typically at slower rates) and merely serves to highlight the difficulty in achieving successful treatment of the cancer in the patient without causing irreparable harm to the healthy cells. As such with many of these treatments there can be devastating side effects which can not only significantly reduce the short term quality of life of the patient but may also have long term detriments on the health of the patient should they survive the cancer attack.
Whilst some of the above problems have substantially been overcome by the development of selective anti-cancer agents (such as tamoxifen) the effectiveness of all chemotherapeutic agents is subject to the development of drug resistance by the cancer cells in the patient. The development of drug resistance in the cancer cells of a patient tends to be class specific and therefore if the cancer cells of a patient develop drug resistance to a class of anti-cancer drugs then all compounds within that class are typically rendered ineffective in the further treatment of that patient. As such in improving clinical outcomes for patients the identification of alternative chemotherapeutic agents is essential in providing the oncologist with an arsenal of drugs that may be used in any given situation.
The development of different classes of therapeutic agents is therefore important as it can help avoid the development of drug resistance and can also be used in combination therapies. Such combination therapies typically involve the use of anti-cancer drugs with different properties and cellular targets which in turn tends to increase the overall effectiveness of any chosen chemotherapy regime and limits the possibility of drug resistance developing in the patient.
One of the major advances in cancer research has been the clinical validation of molecularly targeted drugs that inhibit the activity of protein kinases. Small-molecule kinase inhibitors that are now approved for oncology indications include imatinib, gefitinib, erlotinib, sorafenib, sunitinib and dasatinib [Baselga J., Science, 2006, 312, 1175-1178]. A number of kinases such as JAK2, FLT3 and CDK2 are promising kinase targets for pharmacological intervention in solid tumours, hematological malignancies, myeloproliferative disorders and non-malignant proliferative disorders like keloids.
The Janus kinases (JAK) are a family of cytoplasmic tyrosine kinases consisting of JAK1, JAK2, JAK3 and Tyk2. They play a pivotal role in the signaling pathways of numerous cytokines, hormones and growth factors [Rawlings J S et al, J. Cell Sci., 2004, 117, 1281-1283]. Their intracellular substrates include the family of proteins called Signal Transducer and Activator of Transcription (STAT). The JAK-STAT pathways, through the proper actions of the ligands, regulate important physiological processes such as immune response to viruses, erythropoiesis, lactation, lipid homeostasis, etc. However, dysfunctional signaling caused by a myriad of factors result in pathophysiological conditions such as allergies, asthma, rheumatoid arthritis, severe combined immune deficiency, hematological malignancies, etc. In particular, mutations in JAK2 have been associated with myeloproliferative disorders (including polycythemia vera, essential thrombocythemia and idiopathic myelofibrosis) and a wide range of leukemias and lymphomas [Percy M J et al, Hematol. Oncol., 2005, 23, 91-93]. Importantly, the myeloproliferative disorders belong to an area of unmet medical need where some treatment modalities have not been updated over the past few decades [Schafer A I, Blood, 2006, 107, 4214-4222].
The myeloproliferative disorders (MPDs) belong to a group of hematological malignancies arising from clonal expansion of mutated progenitor stem cells in the bone marrow. The association of one MPD, chronic myeloid leukemia, with the Philadelphia chromosome has been well documented. The Philadelphia negative MPDs include Essential Thrombocythemia (ET), Polycythemia Vera (PV) and Chronic Idiopathic Myelofibrosis (MF). No effective treatment is currently available. The recent discovery that a single acquired somatic mutation in JAK2 appears responsible for many of the features of these MPDs promises to impact the diagnosis and treatment of patients with these disorders and to spur additional research into the origins of dysregulated cell growth and function. Until recently, most MPDs have been considered to be rare or orphan diseases but studies underway suggest a much higher prevalence.
Essential Thrombocythemia is a chronic MPD characterized by an increased number of circulating platelets, profound marrow megakaryocyte hyperplasia, splenomegaly and a clinical course punctuated by hemorrhagic or thrombotic episodes or both. Current treatment options include low dose aspirin, or platelet lowering agents such as anagrelide, interferon or hydroxyurea. These treatments have severe side effects that compromise the quality of life of patients.
Polycythemia Vera is a chronic progressive MPD characterized by an elevated hematocrit, an increase in the red cell mass, and usually by an elevated leukocyte count, an elevated platelet count and an enlarged spleen. The most common cause of morbidity and mortality is the predisposition of PV patients to develop life threatening arterial and venous thromboses. Treatment options include: phlebotomy with low dose aspirin or myelosuppressive therapy options such as hydroxyurea, interferon or anagrelide. Again, these treatments are not ideal due to severe side effects.
Chronic Idiopathic Myelofibrosis (MF) is a chronic malignant hematological disorder characterized by an enlarged spleen, varying degrees of anemia and low platelet counts, red cells in the peripheral blood that resemble tear drops, the appearance of small numbers of immature nucleated red cells and white cells in the blood, varying degrees of fibrosis of the marrow cavity (myelofibrosis) and the presence of marrow cells outside the marrow cavity (extramedullary hematopoiesis or myeloid metaplasia). Current treatment is directed at alleviation of constitutional symptoms, anemia and symptomatic splenomegaly. Treatment options include hydroxyurea, interferon, thalidomide with prednisone, and allogeneic stem cell transplant. MF has the worst prognosis among the Philadelphia negative MPD and represents an area of greatest unmet medical need.
In addition, due to its role in the angiotensin II signaling pathway, JAK2 is also implicated in the etiology of cardiovascular diseases like congestive heart failure and pulmonary hypertension [Berk B C et al, Circ. Res, 1997, 80, 607-616]. Furthermore, a putative role for JAK2 has been demonstrated in keloid pathogenesis and may constitute a new approach for keloid management [Lim C P et al, Oncogene, 2006, 25, 5416-5425]. Yet another potential application for JAK2 inhibitors lies in the treatment of retinal diseases as JAK2 inhibition was found to offer protective effects on photoreceptors in a mouse model of retinal degeneration [Samardzija M et al, FASEB J., 2006, 10, 1096].
A family of Class III receptor tyrosine kinases (RTK), including c-Fms, c-Kit, fms-like receptor tyrosine kinase 3 (FLT3), and platelet-derived growth factor receptors (PDGFRα and β), play an important role in the maintenance, growth and development of hematopoietic and non-hematopoietic cells. Overexpression and activating mutations of these RTKs are known to be involved in the pathophysiology of diverse human cancers from both solid and hematological origins [Hannah A L, Curr. Mol. Med., 2005, 5, 625-642]. FLT3 mutations were first reported as internal tandem duplication (FLT3/ITD) of the juxtamembrane domain-coding sequence; subsequently, point mutations, deletions, and insertions surrounding the D835 coding sequence have been found [Parcells B W et al, Stem Cells, 2006, 24, 1174-1184]. FLT3 mutations are the most frequent genetic alterations reported in acute myeloid leukemia (AML) and are involved in the signaling pathway of autonomous proliferation and differentiation block in leukemia cells [Tickenbrock L et al, Expert Opin. Emerging Drugs, 2006, 11, 1-13]. Several clinical studies have confirmed that FLT3/ITD is strongly associated with a poor prognosis. Because high-dose chemotherapy and stem cell transplantation cannot overcome the adverse effects of FLT3 mutations, the development of FLT3 kinase inhibitors could produce a more efficacious therapeutic strategy for leukemia therapy.
Cyclin-dependent kinases (CDKs) are serine-threonine kinases that play important roles in cell cycle control (CDK1, 2, 4 and 6), transcription initiation (CDK7 and 9), and neuronal function (CDK5) [Knockaert M et al, Trends Pharmacol. Sci., 2002, 23, 417-425]. Aberrations in the cell cycle CDKs and their cyclin partners have been observed in various tumour types, including those of the breast, colon, liver and brain [Shapiro G I, J. Clin. Oncol., 2006, 24, 1770-1783]. It is believed that the pharmacological inhibition of CDK1, 2, 4, 6 and/or 9 may provide a new therapeutic option for diverse cancer patients. In particular, the simultaneous inhibition of CDK1, 2 and 9 has recently been shown to result in enhanced apoptotic killing of lung cancer (H1299) and osteosarcoma cells (U2OS), compared with inhibition of single CDK alone [Cai D et al, Cancer Res, 2006, 66, 9270-9280].
Accordingly, compounds that are kinase inhibitors have the potential to meet the need to provide further biologically active compounds that would be expected to have useful, improved pharmaceutical properties in the treatment of kinase related conditions or disorders such as cancer and other proliferative disorders.